Variable duty cycle swtiching circuit

ABSTRACT

A magnetically coupled multivibrator is normally prevented from running free due to the presence of a forwardly biased transistor which holds the base electrode of one of the two multivibrator transistors below its level of base-emitter conduction. A variable current source charges a capacitor to a voltage level which cuts off the holding transistor and allows the multivibrator circuit to operate for one cycle, during the latter part of which cycle the capacitor is dumped so that the successive cycles are separated in time by that time necessary to charge the capacitor to the required level and which time is variable according to the charging current. The variable duty cycle thus obtained for the multivibrator circuit controls a voltage or power source whose output is fed back inversely to control the charging current so that a regulated voltage or power output is produced.

United States Patent Church 1 June 20, 1972 [54] VARIABLE DUTY CYCLESWTICHING CIRCUIT Richard D. Church, Candor, N.Y.

American Standard Linear Systems, Inc., Candor, N .Y.

[22] Filed: June 25,1970

[21] Appl.No.: 49,802

[72] Inventor:

[73] Assignee:

[52] U.S.Cl ..3l5/lZ7,315/2l9, 321/18,

321/45 R, 323/22 T, 331/113 A [51] Int. Cl ..G05f 1/58 [58] FieldoiSearch ..315/94, 105,106, 119, 127,

315/129, 206, 219, 200 R; 321/2, 18, 45 R, 47; 331/113 A, 66,108 R;323/18, 22 T, 38

[56] References Cited UNITED STATES PATENTS 2,954,531 9/1960 Johnson..331/113 A 2,968,738 l/l96l Pintell ..323/22 T 2,989,686 6/1961Pinckaers et 211..

3,183,430 Schonholzer .....321/47 3,223,909 12/1965 Sensing et a1..323/22 T 3,229,158 l/l966 Jensen 3,310,727 3/1967 Flannery ..321/2 XPrimary Examiner-A. D. Pellinen Attorney-Bean & Bean 57] ABSTRACT Amagnetically coupled multivibrator is normally prevented from runningfree due to the presence of a forwardly biased transistor which holdsthe base electrode of one of the two multivibrator transistors below itslevel of base-emitter con duction. A variable current source charges acapacitor to a voltage level which cuts off the holding transistor andallows the multivibrator circuit to operate for one cycle, during thelatter part of which cycle the capacitor is dumped so that thesuccessive cycles are separated in time by that time necessary 1 tocharge the capacitor to the required level and which time is variableaccording to the charging current. The variable duty cycle thus obtainedfor the multivibrator circuit controls a voltage or power source whoseoutput is fed back inversely to control the charging current so that aregulated voltage or power output is produced.

16 Claims, 4 Drawing Figures PATENTEDmzo I972 SHEET 2 0F 3 INVENTORRICHARD D. CHURCH BY 51M) ATTORNEYS VARIABLE DUTY CYCLE SWTICHINGCIRCUIT BACKGROUND AND BRIEF SUMMARY OF THE INVENTION The systemaccording to this invention essentially involves the use of a pair ofsemiconductor devices inductively coupled by means of a saturable coretransformer and forming a multivibrator biased in such a fashion aswould normally render it free running. By simple control circuitry,however, the multivibrator is allowed to perform only one cycle at atime, the control circuitry being variable in its control to pennit theduty cycle of the multivibrator output to vary between and nearlyIOOpercent. Thus, the system may be used linearly to control the voltageand/or current applied to a load from zero to near maximum.

Variable duty cycle arrangements are known, as for example thosedescribed in U.S. Pat. Nos. 2,813,244 and 3,223,909, as well asreferences cited and referenced therein and, as well, saturable coremagnetically coupled multivibrators are known for power supply control,for example, see US Pat. No. 2,848,614. However, variable duty cyclearrangements normally rely upon some means for controlling pulse widthto obtain the duty cycle variation and, in general, tend to requirerather complex circuitry, whereas regulated power supplies employingsaturable core multivibrators also tend toward complexity and requirefiltering arrangements.

According to the present invention, a pair of transistors operating inconjunction with a saturable core transformer form a multivibrator, anda control circuit arrangement connected thereto imparts an operation tothe circuit which has the characteristics of a monostable binary device.By controlling the periodicity of the triggering action on thismonostable circuit, variable time separation between successive cyclesof action is achieved which allows the system when used as a powercontrol to obtain a duty cycle which may be varied linearly between zeroto nearly unity.

The multivibrator control involves a combination of a variable currentsource and a capacitor charged thereby. Charging of the capacitor tosome predetermined voltage level triggers the multivibrator action for acycle, the latter part of the cycle being used to control a dumpingswitch for thecapacitor so as to obtain the monostable characteristic.By using feedback to control the current source, a regulated poweroutput control is obtained. 3

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified diagramillustrating basic principles according to the present invention;

FIG. 2 is a circuit diagram showing a preferred embodiment of FIG. 1;

FIG. 3 is a circuit diagram showing the use of the circuit of FIG. 2 asan unfiltered voltage regulator; and,

FIG. 4 is a form diagram showing certain principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION The diagrammatic illustration ofFIG. 1 will illustrate the principles according to the presentinvention. As shown, a pair of semiconductor switching devices indicatedgenerally by the reference characters and 12 are connected to asaturable core transformer indicated generally by the referencecharacter 14 in such a manner that by virtue of the biasing of thebase-emitter junction of the switching device 10 across the terminals 16and 18, this switching device will tend to conduct collector currentfrom the terminal 20 and through the winding 23 to the terminal 18which, as will be described hereinbelow, would otherwise initiate freerunning of the multivibrator. A switch 22 or clamping device, however,is provided normally to hold the base electrode of the switching device10 at the potential of the terminal 18 thereby preventing operation ofthe multivibrator circuit. This holding or clamping switch 22 is underthe control of a circuit 24 which operates periodically to disable theswitch 22 and thus allow the switch 10 to conduct collector current.

When the switch 10 conducts collector current, which is caused bycurrent through the winding 26 of the saturable core transformer 14 andthrough the winding 23 of transformer 30 and the load on the windings 32and 34 being reflected to the winding 23, the dot polarity on thewinding 23 becomes positive and the collector-emitter voltage of theswitching device 10 approaches a saturation value. At the same time thedot polarity of the saturable core transformer 14 is also positive sothat the winding 36 reinforces the baseemitter current of the switchingdevice 10 to assure that it is securely conducting collector currentand, at the same time, the winding 38 produces negative voltage on thebase-emitter junction of the switching device 12 so that it may notconduct collector current.

It will be appreciated that the core 40 of the saturable coretransformer is constructed of essentially rectangular hysteresis loopmaterial so that the voltage on the winding 26 remains essentiallyconstant as a function of time until the volt-second product of the corematerial 40 has been satisfied such that the voltage on the winding 26then decreases and passes through 0 volts ultimately to become ofnegative dot polarity. This established a reversal of current throughthe windings 36 and 38 causing the switching device 10 to terminateconduction of collector current and forcing the switching device 12 tocommence conduction of collector current. The control circuit 24 isconnected by conductor 42 to the control conductor 44 on the switchingdevice 12 so that when the latter is biased at its base electrode toconduct collector current, the control circuit 24 operatesto close theswitching device 22 thereby preventing a second cyclic operation of themultivibrator device.

Within the above framework, it will be appreciated that the transformer30 may control a load system to provide for example a power inputthereto which is a function of the periodicity to which themultivibrator circuit is excited to conduct successive cycles ofoperation. Therefore, if the control circuit 24 is actuated to excitethe multivibrator circuit in an increasingly rapid manner, the dutycycle of its output will approach unity. Of course, if the controlsystem 24 does not allow operation of the multivibrator circuit the dutycycle will be zero so that, by proper control, the multivibrator may beoperated between a zero duty cycle characteristic and a duty cyclecharacteristic which approaches unity.

To understand better the characteristics of the system showndiagrammatically only in FIG. 1, reference is had at this time to'thecircuitry shown in FIG. 2; The reference characters utilized for thecircuit components described in conjunction with FIG. 1 are used,wherever possible, in the circuitry of FIG. 2. In FIG. 2, the holdingswitch 22 will be seen to take the form of a transistor having itscollector electrode connected to the base electrode of the switchingdevice 10 and having its emitter electrode connected to the commonterminal 18 and its base electrode is connected through the resistor 50to the terminal 16. The control circuitry 24 includes the transistor 52,the capacitor 54 and a variable current source (not shown) connected tothe terminal 56, and a dumping switch 58 having its collector-emitterpath connected in parallel with the capacitor 54 and having its baseelectrode biased through the resistor 60 from the conductor 44.

The operation of the device has been described thus generally inconnection with FIG. 1. In quiescent condition, since thecollector-emitter saturationvoltage of the switch 22 is less than thethreshold voltage of the base-emitter of the switching device l0, theholding switch 22 will pass collector current so as to hold the switch10 in ofi condition. Current from a variable current source hereinafterdescribed is applied at the junction 56 so that the voltage on thecapacitor 54 begins to rise, it being appreciated that the initialcondition of the capacitor 54 will prevent collector current from beingpassed by the switch 52 and, likewise, the base electrode of the dumpingswitch 58 is biased to prevent collector current from being conducted bythis dumping switch. As soon as the voltage across the capacitor 54 hasreached some predetermined value, the switch 52 will conduct collectorcurrent which, causing a voltage drop across the resistor 50, will causethe switch 22 to cease conduction of collector current, thereby allowingthe switch to pass collector current and thereby initiate a cycle ofoperation as previously described. During the second half of the cycleof operation, that is, when the switching device 12 is biased to passcollector current, the dumping switch S8 likewise is biased to passcollector current which will shunt the capacitor 54 so as to dump thecharge thereon and allow the voltage thereacross to drop so that theswitching device 52 no longer conducts collector current. As a result,the switch 22 will once again conduct and prevent the second cycle ofoperation from occurring until the voltage across the capacitor 54 hasonce again risen to that predetermined value which allows the switch 52to pass collector current. Thus, it will be evident that the timeseparation between successive cycles of operation of the multivibratordevice will depend upon the time required for the capacitor 54 to storea current charge causing the voltage thereacross to achieve theaforementioned predetermined value which closes the switching device 52.Thus, by providing a variable current to the terminal 56, the duty cycleof the circuitry of FIG. 2 can be controlled.

To illustrate a practical embodiment of the invention as thus fardescribed, reference is had to FIG. 3 which shows an unfiltered voltageregulator system. From FIG. 3, it will be appreciated that the output ofthe transformer 30 across its windings 32 and 34 is used alternately tocontrol the semiconductor switches 70 and 72 whereby to allow current toflow from the source 74 to the load 76 alternately through the switches70 and 72 during each cycle of operation of the multivibrator devicedescribed in conjunction with FIG. 2. The voltage output is fed backover the conductor 80 to supply current to an integrating circuitcomprising the resistor 82 and the capacitor 84 connected in series asshown and the juncture between which is connected to the base electrode86 of a current-supplying transistor 88 whose emitter electrode isconnected to the moveable tap 90 of a voltage dividing resistor 92connected across the voltage of the supply 74. Positioning of themovable tap 90 controls the power output to the load 76 since thetransistor 88 will conduct current for charging the capacitor 54inversely with respect to the output voltage at the conductor 80.

The invention as shown in FIG. 3 may be used to provide a particularlyuseful hand-held variable intensity lantern in which, as shown, theoutput load 76 is a suitable light source. The DC. power supply 74 maybe in the form of rechargeable nickel-cadmium batteries. The entiredevice may be of very light weight so as to provide an extremelyportable light source whose intensity may be varied over a wide range.The characteristics of conventional nickel-cadmium batteries allow forprolonged high-intensity illumination whereas the intensity controlallows the light source to be operated at whatever level is required atthe moment. Thus, by providing a manual control for the movable tap 90,the operator may reduce the intensity level to be no greater than thatwhich is required at one instant, thereby reducing the drain on thebatteries, while at the same time being able instantly to increase theintensity to obtain the amount of illumination required by changingcircumstances. Military, underwater and fire fighting uses are examplesof those uses to which the device may be put with advantage.

For this application of the circuit, it is desirable to prevent thevoltage of the battery supply 74 from dropping below a predeterminedlevel and, to this end, a protecting circuit is used in the FIG. 3embodiment. This circuit is not normally operative but will respond tomodify the action when the voltage of the supply drops below apredetermined value.

This function is achieved by the resistor chain 94, 96 and interveningzener diode 98, the values of the resistors 94 and 96 being such thatabove some predetermined voltage level of the supply 74, the zener diode98 will conduct current but will not conduct below the voltage value.The value of the resistor 94 is chosen such that with the diode 98conducting, the voltage at the base electrode 100 of the transistor 102allows this transistor to be in saturation whereby the emitter-base ofthe transistor 104 is not forward biased due to the drop across theresistor 105. As a consequence, no collector current flows through thetransistor 104 and the resistor 106 and the dumping switch 58 operate asis described above.

However, should the voltage of the supply 74 drop to that value whichwill not sustain conduction of the zener diode 98, the transistor 102 iscut off and the emitter base of the transistor 104 is now forward biasedthrough the resistor 105. The resultant current through the resistor 106due to collector current conduction of the transistor 104 causes part ofthe supply current through the resistor 108 to the capacitor 54 to beshunted by the transistor 58 so that the voltage build up across thecapacitor 54 cannot reach that value sufficient to forward bias thetransistor 52. As a result, the duty cycle of the system is abruptlydecreased to zero.

The resulting decrease in power output from the battery supply 74 allowsits output voltage to increase until the zener diode 98 again conductsthereby to cut off the transistor 104. With only those circuitcomponents thus far described, the protection circuit operates to modifythe normal duty cycle as a function of the threshold value below whichthe zener diode 98 will not conduct and in effect decreases the dutycycle otherwise established by the setting of he movable tap 90.

In view of the fact that the efficiency of the device is greater whenoperating at a higher duty cycle, the capacitor 110 is included in theprotective circuit to allow the device to operate for several cycles atmaximum duty cycle before the transistor 104 is turned on due tocessation of current conduction through the zener diode 98.

The effect of the capacitor 110 is best explained in connection withFIG. 4. In this figure, normal operation is represented by the firstthree pulses P each of whose durations represents the successive ontimes for the two switches 70 and 72 and which represents one cycle ofthe device, the off times between these pulses being controlled by thesetting of the movable tap 90. During this normal operation, the voltageof the supply has decreased below that value which allows conductionthrough the zener diode 98 so that after the capacitor 110 hasdischarged sufficiently through the resistor 94, the transistor 102 willcease conduction so that the transistor 104 is turned on to produce theinterval I (zero duty cycle) as described above. During this interval,the switches 70 and 72 do not conduct and the voltage across thecapacitor 84 decreasewhereas the voltage at the source 74 increases toexceed the threshold for the diode 98. As a result of the decrease ofvoltage across the capacitor 84, the transistor will increase itsconduction toward saturation so that when the supply voltage exceeds thethreshold of diode conduction and the transistor 104, with delay, isturned off, the device will run at a cluster of cycles of maximum dutycycle D, as indicated by the pulse cluster C which will terminate whenthe supply voltage drops below the conduction value for the diode 98and, with delay, turns the transistor 104 What is claimed is:

1. A variable duty cycle switching circuit comprising, in combination,

a saturable core transformer having first and second windings and asaturable core inductively coupling said windings,

a pair of oppositely poled switching transistors having theiremitter-collector paths connected in series across the opposite ends ofsaid first winding and having their base electrodes connected torespective opposite ends of said second winding,

impedance means comprising a third winding connected across the oppositeends of said first winding,

D.C. supply means having one side connected to a point intermediate theends of said third winding and its other side connected between the endsof said second winding and to the juncture between saidemitter-collector paths of said transistors, said one side of saidsupply means also being connected to the base electrode of onetransistor normally to forward bias same to conduct collector currentand produce a voltage drop in one direction across said first windingthrough said impedance means, said first and second windings being ofdot polarity first to reinforce the forward bias of said one transistorand then to reverse dot polarity to forward bias the other transistorwhereby to initiate successive cycles of successive actuations of saidtransistors,

control means connected to said one transistor normally preventingforward biasing of same, said control means including variable means forselectively varying the periodicity with which said one transistor isforward biased, and

said variable means comprising a capacitor, a variable current sourcefor charging said capacitor, and a dumping switch for said capacitoractuated in consonance with forward biasing of said other transistor.

2. The variable duty cycle switching circuit according to claim 1wherein said impedance means comprises the primary winding of atransformer, said transformer having a secondary winding, and powerswitching means controlled by said secondary winding.

3. The variable duty cycle switching circuit according to claim 2wherein said variable current source is controlled by said powerswitching means.

4. An electrical system comprising a load, a DC. power supply for saidload, and means for selectively varying the power supplied to said load,the improvement wherein:

said means comprises a variable duty cycle switching circuit forcontrolling the connection of said power supply to said load, saidvariable duty cycle switching circuit including a saturable coretransformer having first and second windings and a saturable coreinductively coupling said windings, a pair of oppositely poled switchingtransistors having their emitter-collector paths connected in seriesacross the opposite ends of said first winding and having their baseelectrodes connected to respective opposite ends of said second winding,impedance means comprising a third winding connected across the oppositeends of said first winding, one side of said power supply beingconnected to a point intermediate the ends of said third winding and theother side of said power supply being connected between the ends of saidsecond winding and to the junction between the emitter-collector pathsof said transistors, said one side of the power supply being alsoconnected to the base electrode of one transistor normally to forwardbias same to conduct collector current and produce a voltage drop in onedirection across said first winding through said impedance means, saidfirst and second windings being of dot polarity first to reinforce theforward biasing of said one transistor and then to reverse dot polarityto forward bias the other transistor whereby said switching circuittends to operate in successive cycles of successive actuation of saidtransistors, and control means connected to said one transistor normallypreventing forward biasing of same, said control means includingvariable means for selectively varying the periodicity with which saidone transistor is forward biased by said power supply,

said variable means comprising a capacitor, a variable current sourcefor charging said capacitor, and a dumping switch for said capacitoractuated in consonance with forward biasing of said other transistor.

5. The electrical circuit according to claim 4 wherein said impedancemeans comprises the primary winding of a transformer, said transformerhaving a secondary winding, and power switching means controlled by saidsecondary winding.

6. The electrical circuit according to claim 5 wherein said variablecurrent source is controlled by the output to said load.

7. An electrical system comprising a load, a D.C. power supply for saidload, and means for selectively varying the power supplied to said load,the improvement wherein:

said means comprises a variable duty cycle switching circuit forcontrolling the connection of said power supply to said load, saidvariable duty cycle switching circuit including a saturable coretransformer having first and second windings and a saturable coreinductively coupling said windings, a pair of oppositely poled switchingtransistors having their emitter-collector paths connected in seriesacross the opposite ends of said first winding and having their baseelectrodes connected to respective opposite ends of said second winding,impedance means comprising a third winding connected across the oppositeends of said first winding, one side of said power supply beingconnected to a point intermediate the ends of said third winding and theother side of said power supply being connected between the ends of saidsecond winding and to the junction between the emitter-collector pathsof said transistors, said one side of the power supply being alsoconnected to the base electrode of one transistor normally to forwardbias same to conduct collector current and produce a voltage drop in onedirection across said first winding through said impedance means, saidfirst and second windings being of dot polarity first to reinforce theforward biasing of said one transistor and then to reverse dot polarityto forward bias the other transistor whereby said switching circuittends to operate in successive cycles of successive actuation of saidtransistors, and control means connected to said one transistor normallypreventing forward biasing of same, said control means includingvariable means for selectively varying the periodicity with which saidone transistor is forward biased by said power supply,

said load being an illuminating bulb and said power supply beingnickel-cadmium battery means, and a protective circuit means responsiveto the voltage of said battery means for abruptly decreasing the dutycycle of the switching circuit in the event that the voltage of thesupply drops below a predetermined level.

8. The electrical circuit according to claim 7 wherein said protectivecircuit means includes means for periodically operating said switchingcircuit in maximum duty cycle pulse clusters.

9. The electrical circuit according to claim 8 wherein said variablemeans comprises a capacitor, a variable current source for charging saidcapacitor, and a dumping switch for said capacitor actuated inconsonance with forward biasing of said other transistor.

10. The electrical circuit according to claim 9 wherein said impedancemeans comprises the primary winding of a transformer, said transformerhaving a secondary winding, and power switching means controlled by saidsecondary winding.

11. The electrical circuit according to claim 10 wherein said variablecurrent source is controlled by the output to said load.

12. The electrical circuit according to claim 7 wherein said variablemeans comprises a capacitor, a variable current source for charging saidcapacitor, and a dumping switch for said capacitor actuated inconsonance with forward biasing of said other transistor.

13. The electrical circuit according to claim 12 wherein said impedancemeans comprises the primary winding of a transformer, said transformerhaving a secondary winding, and power switching means controlled by saidsecondary winding.

14. The electrical circuit according to claim 13 wherein said variablecurrent source is controlled by the output to said load.

15. The electrical circuit according to claim 7 wherein said impedancemeans comprises the primary winding of a transformer, said transformerhaving a secondary winding, and power switching means controlled by saidsecondary winding.

16. A variable duty cycle switching circuit comprising, in combination,

control means connected to said first semiconductor switching devicenormally preventing said biasing means from closing same, said controlmeans including variable means for selectively varying the periodicitywith which said first semiconductor switching device is permitted toclose;

said variable means comprising a capacitor, a variable current sourcefor charging said capacitor, and a dumping switch for said capacitoractuated in consonance with closing of said second semiconductorswitching device.

1. A variable duty cycle switching circuit comprising, in combination, asaturable core transformer having first and second windings and asaturable core inductively coupling said windings, a pair of oppositelypoled switching transistors having their emitter-collector pathsconnected in series across the opposite ends of said first winding andhaving their base electrodes connected to respective opposite ends ofsaid second winding, impedance means comprising a third windingconnected across the opposite ends of said first winding, D.C. supplymeans having one side connected to a point intermediate the ends of saidthird winding and its other side connected between the ends of saidsecond winding and to the juncture between said emitter-collector pathsof said transistors, said one side of said supply means also beingconnected to the base electrode of one transistor normally to forwardbias same to conduct collector current and produce a voltage drop in onedirection across said first winding through said impedance means, saidfirst and second windings being of dot polarity first to reinforce theforward bias of said one transistor and then to reverse dot polarity toforward bias the other transistor whereby to initiate successive cyclesof successive actuations of said transistors, control means connected tosaid one transistor normally preventing forward biasing of same, saidcontrol means including variable means for selectively varying theperiodicity with which said one transistor is forward biased, and saidvariable means comprising a capacitor, a variable current source forcharging said capacitor, and a dumping switch for said capacitoractuated in consonance with forward biasing of said other transistor. 2.The variable duty cycle switching circuit according to claim 1 whereinsaid impedance means comprises the primary winding of a transformer,said transformer having a secondary winding, and power switching meanscontrolled by said secondary winding.
 3. The variable duty cycleswitching circuit according to claim 2 wherein said variable currentsource is controlled by said power switching means.
 4. An electricalsystem comprising a load, a D.C. power supply for said load, and meansfor selectively varying the power supplied to said load, the improvementwherein: said means comprises a variable duty cycle switching circuitfor controlling the connection of said power supply to said load, saidvariable duty cycle switching circuit including a saturable coretransformer having first and second windings and a saturable coreinductively coupling said windings, a pair of oppositely poled switchingtransistors having their emitter-collector paths connected in seriesacross the opposite ends of said first winding and having their baseelectrodes connected to respective opposite ends of said second winding,impedance means comprising a third winding connected across the oppositeends of said first winding, one side of said power supply beingconnected to a point intermediate the ends of said third winding and theother side of said power supply being connected between the ends of saidsecond winding and to the junction between the emitter-collector pathsof said transistors, said one side of the power supply being alsoconnected to the base electrode of one transistor normally to forwardbias same to conduct collector current and produce a voltage drop in onedirection across said first winding through said impedance means, saidfirst and second windings being of dot polarity first to reinforce theforward biasing of said one transistor and then to reverse dot polarityto forward bias the other transistor whereby said switching circuittends to operate in successive cycles of successive actuation of saidtransistors, and control means connected to said one transistor normallypreventing forward biasing of same, said control means includingvariable means for selectively varying the periodicity with which saidone transistor is forward biased by said power supply, said variablemeans comprising a capacitor, a variable current source for chargingsaid capacitor, and a dumping switch for said capacitor actuated inconsonance with forward biasing of said other transistor.
 5. Theelectrical circuit according to claim 4 wherein said impedance meanscomprises the primary winding of a transformer, said transformer havinga secondary winding, and power switching means controlled by saidsecondary winding.
 6. The electrical circuit according to claim 5wherein said variable current source is controlled by the output to saidload.
 7. An electrical system comprising a load, a D.C. power supply forsaid load, and means for selectively varying the power supplied to saidload, the improvement wherein: said means comprises a variable dutycycle switching circuit for controlling the connection of said powersupply to said load, said variable duty cycle switching circuitincluding a saturable core transformer having first and second windingsand a saturable core inductively coupling said windings, a pair ofoppositely poled switching transistors having their emitter-collectorpaths connected in series across the opposite ends of said first windingand having their base electrodes connected to respective opposite endsof said second winding, impedance means comprising a third windingconnected across the opposite ends of said first winding, one side ofsaid power supply being connected to a point intermediate the ends ofsaid third winding and the other side of said power supply beingconnected between the ends of said second winding and to the junctionbetween the emitter-collector paths of said transistors, said one sideof the power supply being also connected to the base electrode of onetransistor normally to forward bias same to conduct collector currentand produce a voltage drop in one direction across said first windingthrough said impedance means, said first and second windings being ofdot polarity first to reinforce the forward biasing of said onetransistor and then to reverse dot polarity to forward bias the othertransistor whereby said switching circuit tends to operate in successivecycles of successive actuation of said transistors, and control meansconnected to said one transistor normally preventing forward biasing ofsame, said control means including variable means fOr selectivelyvarying the periodicity with which said one transistor is forward biasedby said power supply, said load being an illuminating bulb and saidpower supply being nickel-cadmium battery means, and a protectivecircuit means responsive to the voltage of said battery means forabruptly decreasing the duty cycle of the switching circuit in the eventthat the voltage of the supply drops below a predetermined level.
 8. Theelectrical circuit according to claim 7 wherein said protective circuitmeans includes means for periodically operating said switching circuitin maximum duty cycle pulse clusters.
 9. The electrical circuitaccording to claim 8 wherein said variable means comprises a capacitor,a variable current source for charging said capacitor, and a dumpingswitch for said capacitor actuated in consonance with forward biasing ofsaid other transistor.
 10. The electrical circuit according to claim 9wherein said impedance means comprises the primary winding of atransformer, said transformer having a secondary winding, and powerswitching means controlled by said secondary winding.
 11. The electricalcircuit according to claim 10 wherein said variable current source iscontrolled by the output to said load.
 12. The electrical circuitaccording to claim 7 wherein said variable means comprises a capacitor,a variable current source for charging said capacitor, and a dumpingswitch for said capacitor actuated in consonance with forward biasing ofsaid other transistor.
 13. The electrical circuit according to claim 12wherein said impedance means comprises the primary winding of atransformer, said transformer having a secondary winding, and powerswitching means controlled by said secondary winding.
 14. The electricalcircuit according to claim 13 wherein said variable current source iscontrolled by the output to said load.
 15. The electrical circuitaccording to claim 7 wherein said impedance means comprises the primarywinding of a transformer, said transformer having a secondary winding,and power switching means controlled by said secondary winding.
 16. Avariable duty cycle switching circuit comprising, in combination, asaturable core transformer having a first and second winding; a firstnormally open semiconductor switching device connected to one end ofsaid second winding; a second normally open semiconductor switchingdevice connected to the other end of said second winding; biasing meansconnected to both of said semiconductor devices, said first windingbeing connected at its opposite ends to respective ones of of saidsemiconductor devices whereby said biasing means tends to initiatesuccessive cycles of successive actuation of said semiconductorswitching devices; control means connected to said first semiconductorswitching device normally preventing said biasing means from closingsame, said control means including variable means for selectivelyvarying the periodicity with which said first semiconductor switchingdevice is permitted to close; said variable means comprising acapacitor, a variable current source for charging said capacitor, and adumping switch for said capacitor actuated in consonance with closing ofsaid second semiconductor switching device.